Metal nitride coatings are known to have a number of chemical and physical properties which make their use desirable in myriad technical applications. Such nitrides are generally hard, refractory materials and may be useful, for example, in such diverse applications as wear-resistant coatings for cutting tools, superconductive films, diffusion barriers and gates in integrated circuits, and in solar control panels.
Metal nitride films may be prepared in several ways. For example, a metal target located near the substrate to be coated may be subjected to plasma containing nitrogen. The corresponding metal nitride is then deposited onto the substrate. This method suffers from the drawbacks of slow and uneven deposition, more particularly a problem when large substrates are to be coated. Like other "line of sight" deposition methods, this method exhibits poor conformal coverage on substrates with high three dimensional character. This poor conformal coverage is particularly a problem in the electronics industry where integrated circuits, for example, have numerous peaks and valleys which require uniform coverage. Moreover, some substrates are sensitive to the high energy environment and thus cannot be coated in this way. Finally, such techniques require the use of relatively high vacuum apparatus, a concern of special importance with physically large substrates.
Laser ablation techniques may also be used. In this case, a target of metal nitride is a ablated in vacuuo by a laser beam, the ablated metal nitride then forming a coating on the substrate. Unfortunately, the ablation process results in generation of clusters of metal nitride particles containing from 2 to 100 metal nitride units. Smooth, highly crystalline coatings are difficult to prepare with this method. Like other "line of sight" physical deposition methods, laser ablation suffers from the drawback of poor conformal coverage.
Chemical vapor deposition (CVD) processes generally exhibit excellent conformal coverage, and have been used to prepare metal nitride coatings. For example, the CVD reaction of niobium pentachloride with ammonia and hydrogen at 950.degree.-1000.degree. C. affords NbN coatings, as reported by G. I. Oya and Y. Onodera, J. APPL. PHYS., 45, 1389 (1974). The high temperatures used in this process precludes use in many applications, for example in integrated circuit coatings.
Treatment of Nb(NR.sub.2).sub.5 with ammonia at 200.degree.-450.degree. C. at atmospheric pressure has provided NbN coatings according to R. Fix et al., CHEM. MATER., 5 614 (1993). However, this method, like that of Oya, requires two reactants, a variation in the relative concentrations of which can affect the stoichiometry of the coating. Fix et al. also disclose preparation of Ta.sub.3 N.sub.5 coatings. Such coatings have also been prepared by the gas-phase ammonolysis of tantalum pentachloride at 600.degree.-1000.degree. C. as reported by K. Hieber, THIN SOLID FILMS, 24, 157 (1974). The foregoing methods, however, are not suitable for the preparation of the often preferred TaN coatings, which may be achieved through the CVD reaction of tantalum pentachloride, nitrogen, and hydrogen in the temperature range of 700.degree.-1000.degree. C., as reported by T. Takahashi et al., J. LESS COMMON MET., 52, 29 (1977). Once again, the high temperatures involved, as well as the necessity for three reactant streams, render such processes problematic.
It would be most desirable to be able to obtain metal nitride coatings from but a single precursor by CVD. The use of a single precursor obviates the need for separate reactant streams, and the accompanying necessity to carefully monitor the concentrations of these streams. H. T. Chiu et al., in J. MAT. Sci. LETT., 11, 96 (1992), reported that the complex [(Et.sub.2 N).sub.3 Ta(.eta..sup.2 -EtN=CMeH)] serves as a single source precursor to TaN coatings between 500.degree.-650.degree. C. K. Sugiyama et al., J. ELECTROCHEM. SOC., 122, 1545 (1975) has described deposition of tantalum nitride coatings of unspecified composition from the single source precursor [Ta(NEt.sub.2).sub.5 ]. There have been no known reports of single source precursors to niobium nitride coatings.
U.S. Pat. No. 5,194,642 discloses formation of coatings from single source precursors of the formula [TiCl.sub.2 (NR)(NH.sub.2 R).sub.2 ].sub.3. However, it is desirable to identify other single source precursors for metal nitride coatings, and particularly, to provide precursors which are able to prepare tantalum nitride coatings having TaN stoichiometry. It is more particularly desirable to identify precursors which may be used to prepare such coatings at low temperatures so as to have widespread applicability as well as economy in application.